An integrated circuit electronic package can be fabricated by applying to a ceramic substrate or carrier a chrome layer followed by a copper layer followed by another chrome layer. Next, a photoresist composition is applied so that selected areas of the chrome/copper/chrome layers can be removed to provide the desired electrical connections on the substrate. The top chrome layer in predetermined areas is removed by known etching techniques. The top chrome layer is present so that subsequently applied solder will not adhere to the substrate in those areas where the chrome remains.
Next, exposed copper is removed from preselected areas wherein it is desired to include solder, such as around the holes wherein pins are to be subsequently inserted, and at the places where the semiconductor chip(s) are to be interconnected by solder with the substrate. The copper layer provides electrical conductivity.
Next the exposed areas of the bottom layer of chrome are removed by etching. The bottom chrome layer is employed to provide adequate adhesion between the copper layer and the ceramic substrate.
After the input/output pins are inserted into the substrate, solder is applied to the substrate and will deposit on the input/output pins and the exposed copper regions, but not on the substrate itself and not on the exposed chrome areas.
A problem with this particular technique is that the solder when applied forms ball- or sphere-like deposits on the exposed areas. Such, however, are of uneven height. This is significant in the regions where the chip is to be connected to the substrate. Such regions are commonly referred to in the art as the chip pad area. The uneven deposits in the chip pad area can cause electrical shorts and early thermal cycle (fatigue) failures. Solder is also applied to the integrated circuit chips but such is of a fairly uniform and even coating since it is applied by evaporation techniques. However, evaporation techniques are not especially suitable for applying solder to the substrates since evaporation techniques are relatively expensive and too slow for high volume production. Also, the high temperatures employed in evaporation techniques are likely to destroy photomasks while the solder is being deposited resulting in the photomask not performing its intended function of defining the areas where the solder is to be deposited.
The above problems with respect to the prior art are obviated by the present invention whereby certain compositions are applied to the chip pad area to protect it from having the solder deposit on the exposed copper. Accordingly, upon removal of the composition, the only solder present which is needed to connect the chip and substrate is that on the chip which is, as discussed hereinabove, present as an even coating. This results in uniform height and increased reliability of the packages so made. Also, the fact that no solder was initially present on the copper and the chip pad area does not in any way reduce the strength of the bond between the chip and substrate.
For a composition to be suitable for the above purpose, it must possess a number of characteristics, some of which seem to be contradictory to other needed characteristics of the composition. For instance, the composition must be readily accurately applied to the desired area to be protected from the solder. This requires that the composition have certain flow characteristics.
The composition must be resistant to the solder at the conditions at which the solder is applied to the substrate. Moreover, the composition must be sufficiently adherent to the substrate so that the solder will not seep or flow underneath the composition and contact those copper areas which are to be protected. However, contrary to this property of adherence to the substrate, it is necessary that the composition not so tenaciously adhere to the substrate that subsequent removal of the composition is impractical and/or destructive to the substrate or other layers thereon. Accordingly, the composition must be readily strippable, yet adequately adhere to the substrate to be suitable for the purposes to which the present invention are directed.
Providing compositions containing all of the above properties is quite difficult. The difficulties in finding suitable compositions is further magnified by the fact that integrated circuits are getting more complex and that various patterns on the substrates are getting closer together and are becoming more difficult to follow. Therefore, the patternable properties of a composition for such purpose are very demanding.
The above objects have been accomplished by the present invention which provides a composition containing a polyepoxide or polyimide/amide, certain oil detackifiers, and high temperature resistant fillers in particular relative amounts. The detackifiers include liquid silicone oils, drying oils, terpenes and terpineols.
The prior art does not suggest the compositions of the present invention and does not suggest that such would possess the critical combination of properties necessary to permit the compositions to be used as a soldering mask in the type of process discussed hereinabove. For instance, U.S. Pat. No. 3,288,754 to Green suggests reacting a reactive or functional silane with a polyimide/amide. On the other hand, as discussed hereinabove, the present invention requires a mixture which can include a silicone material with a polyamide/imide rather than a reaction product. Also, as will be discussed hereinbelow, the silicone when employed does not substantially react with the polyamide/imide but instead during the process vaporizes. Furthermore, the composition of the present invention employs certain quantities of particular types of fillers which are not suggested in U.S. Pat. No. 3,288,754.
U.S. Pat. Nos. 3,843,577 and 3,926,885 to Keil, although suggesting compositions containing a polyepoxide and certain specific siloxanes, fail to suggest the present invention since, among other things, these patents require the presence of an incompatible lubricant and employ a curing agent. Furthermore, these patents do not suggest the types of properties achieved by the particular compositions of the present invention, and, in fact, such properties seem to be contrary to those desired by said patent particularly because of the presence of the curing agents.
U.S. Pat. No. 4,121,000 to Wald, although suggesting a composition which can contain an epoxy polymer and a polysiloxane fluid, fails to teach the present invention since, among other things, such patent requires the presence of a large amount of other polymeric materials in the composition as well as hardening agents.
U.S. Pat. No. 3,816,364 to Bayer, although suggesting certain compositions which contain an epoxy and a dimethyl silicone, fail to teach the present invention since, among other things, such compositions require solid epoxides and do not suggest the same amounts of silicone as are required by the present invention.
U.S. Pat. No. 3,781,237 to Alvino et al suggests a composition which contains a polyimide/amide and a polysiloxane. However, such patent fails to suggest the present invention since, among other things, the quantity of polysiloxane required by said patent is significantly less than that employed according to the present invention.
U.S. Pat. No. 3,440,203 to Boldebuck et al also suggests a composition which contains a polyimide and a polysiloxane. However, this patent fails to suggest or render obvious the present invention since the amount of polysiloxane required by this patent is significantly less than that employed according to the present invention.
U.S. Pat. No. 3,556,754 to Marsden et al suggests compositions which contain a specific type of silicone and a polymer which can be an epoxy for use in sizing glass fibers. However, this patent fails to suggest the present invention since, among other things, the amount of silicone suggested is much less than that employed according to the present invention.
U.S. Pat. No. 3,305,504 to Huntington suggests a composition which contains an epoxy and a silicone gum. This patent fails to suggest the present invention since, among other things, it requires a silicone gum whereas the present invention employs a silicone oil.